One back in public house left some disinfectant (6)
I believe the answer is:
phenol
'some disinfectant' is the definition.
(thesaurus)
'one back in public house left' is the wordplay.
'back' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'public house' becomes 'ph'.
'left' becomes 'l' (common abbreviation**).
'one' written backwards gives 'eno'.
'ph'+'l'='phl'
'eno' inserted inside 'phl' is 'PHENOL'.
(Other definitions for phenol that I've seen before include "cleaner" , "Powerful disinfectant" , "Carbolic acid" , "Acidic substance" , "Organic compound; no help (anag.)" .)